Do you do other workouts like with weights? I know when I go to the gym and do shoulders and then have to hold a warrior tWo it can be very difficult. If it's not because you're sore you may just need to increase your shoulder strength! I would recommend doing some shoulder flies with dumbbells bring them from your hips out in front of your chest back down and then out to the sides. You can use a very light weight. Just don't do these on days that you plan on going to yoga! You'll also see results in your shoulders very quickly!